During the second year of support we examined the functional significance of low ohmic resistance specialized Purkinje fiber-muscle junctions between Purkinje fibers of the proximal left bundle branch and ventricular myocardial cells located in the left basal portion of the interventricular septum. The study confirmed our preliminary observation that these junctions were of low resistance allowing close electrical coupling. When the bulk of muscle in this region was connected to the proximal left bundle branch, the action potential duration of the bundle was 293 plus or minus 17 msec. When the muscle was disconnected by transection of the bulk of the junctions, left bundle action potential duration prolonged to 329 plus or minus 29 msec (p less than 0.01). This observation indicated that electrical interaction across these intact nexuses effected significant foreshortening of proximal left bundle branch action potential and refractory period durations. Pilot studies were performed during this year of support to test the feasibility of temporarily increasing the ohmic resistance between the cardiac cells by the superfusion of hypertonic solutions. Using conventional microelectrode techniques we measured the effects on canine cardiac Purkinje fibers of Tyrode's solution rendered hypertonic by the addition of sucrose to raise the osmolality from a control of 300 to 350, 400, and 500 mos. Our preliminary results indicate that hypertonic solutions may exert a significant antiarrhythmic effect. In the coming year we plan experiments that will further elucidate the antiarrhythmic effects of hypertonic solutions and will try to correlate these antiarrhythmic properties with electrical uncoupling. BIBLIOGRAPHIC REFERENCES: Bailey, J.C., Lathrop, D.A., Besch, H.R., Jr., and Watanabe, A.M.: Cholinergic antagonism of adrenergic effects on cardiac Purkinje fibers. Clin. Res. 25:206A, 1977. Elharrar, V., Bailey, J.C., Lathrop, D.A., and Zipes, D.P.: Effects of aprindine HCl on slow channel action potentials and transient depolarizations in canine Purkinje fibers. Fed. Proc. 36:783, 1977.